In three-phase motors having a stator and a rotor, the ratio between stator poles and rotor poles is always unequal. For example, a three-phase motor having six stator poles that are arranged at a spacing of 60° from one another, and has four rotor poles that are arranged at a spacing of 90° from one another. Each of the three phases here has two mutually oppositely located stator poles associated with it. The result is that, in any rotor position, it is possible to generate a torque via at least two of the three phases, since a magnetically different position of the rotor poles is located opposite each of the three phases. It is thus possible to start the motor with high torque in any rotor position.
With single-phase motors, the number of rotor poles corresponds to the number of stator poles. The consequence of this is that whenever each rotor pole is located opposite exactly one stator pole, no torque can be generated by a flow of current through the winding arrangement. In addition, for example in fans, the rotor preferentially assumes precisely that position when stopped, since it corresponds to the lowest-energy state.
Several methods are already known for enabling starting of such a motor, despite this. DE 8 702 271 U1, corresponding to SCHIMIDER U.S. Pat. No. 5,109,171, describes an electric motor in which soft magnetic iron plates are mounted on the rotor in such a way that in the zero-current-flow state, it assumes a position from which starting is possible. This is also referred to as generating an “auxiliary reluctance torque.”
DE 3 149 766 A1, corresponding to MUELLER U.S. Pat. No. 4,730,136, describes an electric motor in which the stator poles are implemented asymmetrically, so that the spacing between the stator poles and the rotor decreases as viewed in a circumferential direction. The rotor poles of the rotor magnet preferentially assume a position in which they are on average as close as possible to the stator pole. This once again results, in the zero-current-flow state, in a starting position from which the motor can begin to operate.
Millions of such motors are used in CD players and hard drives.
The aforesaid approaches require a motor in which the external torque and the friction that occur are not too great, as is the case e.g. with fans. Otherwise there is no guarantee that the starting position can reliably be assumed. The area of application for these approaches is thus limited.